Chapter XLII: Marine Engines (1)

Marine engines are made in the following forms:

1. With a single or with two cylinders receiving live steam from the boilers, and exhausting into the atmosphere. These are termed high pressure engines, let the steam pressure be what it may. They are also, and more properly, termed non-condensing engines.

In the small sizes, such as are used for launch engines, it is simply a non-condensing engine, with a link motion for varying the point of cut off as well as for reversing purposes. Fig. 3392 represents an engine of this class constructed by Chas. P. Willard & Co.

The cylinder is what is called "inverted," meaning that it is above the crank shaft.

The slide spindle or valve rod passes through a guide and connects direct to the link block or die, as it is sometimes called.

The thrust block is provided in the bearing of the crank shaft, and consists, as seen in the sectional view, of a series of collars on the crank shaft bearing.

2. The addition to each high pressure cylinder of a low pressure cylinder constitutes a compound engine, and if the engine has also a condenser, it is a compound condensing engine, an example being shown in Fig. 3392_a_, which represents an engine in which the link motions are employed to vary the points of cut off of both cylinders, as well as to reverse the engine. The engine being small, the power required to move the links is small enough to permit of their operation by hand, by means of the hand lever L, which is secured to its adjusted position on the sector T by the small lever nut shown on the side of the lever. The lever L operates a shaft D which shifts both link motions. The air and circulating pumps are at the back of the condenser, being operated from the beams B, B, each beam connecting to rods J which connect to rod _c_, which drives the air and circulating pumps.

The steam from the high pressure cylinder exhausts into a receiver or chamber between the two cylinders, and from which the low pressure cylinder receives its steam.

The exhaust from the low pressure cylinder passes into the condenser, where it is condensed, leaving a partial vacuum on the exhaust side of the low pressure piston.

Figs. 3393 and 3394 show the arrangement of the pumps on a pair of compound engines for a dredger. The steam from the low pressure cylinder passes into the body of the condenser with which the air pump is in communication, as shown in the end elevation. At _a_ is the foot valve of the condenser. The piston of the air pump has a similar valve, and at _e_ is the delivery valve.

The circulating pump is shown in the back elevation (Fig. 3394), being a piston pump which forces the water through the tubes of the condenser.

There are two principal methods of compounding, in one of which the two cylinders are placed one above the other, with their axes in line, and both pistons connecting to the same crank, while in the other the cylinders are side by side, and each connects to its own crank, the two cranks usually being at a right angle.

When one cylinder is placed above the other, as in Fig. 3395, R being the high pressure and S the low pressure piston, no receiver is employed, the steam passing direct from the high pressure cylinder through the pipe P to the low pressure steam chest _c_. The high pressure steam valve V and the low pressure valve V are on the same stem, a cut off valve V´ being provided for the high pressure cylinder.

3. Triple expansion engines have three cylinders, a high pressure, an intermediate, and a low pressure cylinder.

In a triple expansion engine the intermediate cylinder receives the steam that is exhausted from the high pressure cylinder, and expands it further. The low pressure cylinder receives its steam from the exhaust of the intermediate cylinder, and exhausts into the condenser.

In the illustrations from Fig. 3396 to Fig. 3406 are represented the triple expansion engines of the steamship _Matabele_, constructed by Messrs. Hall, Russell & Company, of Aberdeen, Scotland. Fig. 3396 is a cross sectional view of the vessel showing the engine and its connections, and Fig. 3397 a similar view, showing the boilers. Fig. 3398 is a back elevation of the engine, showing the boilers also, and Fig. 3399 a plan of the same. Fig. 3400 is a sectional view, and Fig. 3401 an end view of the boilers. Fig. 3402 is a plan, Fig. 3403 an end elevation, and Fig. 3404 a front elevation, partly in section, of the engines. H P is the high pressure cylinder, I C the intermediate cylinder, and L P the low pressure cylinder. The high pressure cylinder has a piston valve, the steam chest being shown at A. The intermediate cylinder is provided with a double ported flat valve as shown at B, and the low pressure cylinder is provided with a similar valve whose weight is counterbalanced by the small piston at E; at F are the relief valves for relieving the cylinders of water.

Fig. 3393.

Fig. 3394.]

Each steam valve is provided with a link motion that may be used for varying the point of cut off (and therefore the expansion) as well as for reversing purposes.

The link motions are all shifted from one shaft, which may be operated by hand or by steam, the construction being as follows:

For shifting by hand, the wheel W is operated, its shaft having a worm driving the worm wheel G, Fig. 3403, which operates rod H, and through the lever J and rod K shifts the link L, one pair of eccentric rods being shown at N and P.

The shaft of the wheel W is, however, a crank shaft, and at M is a small engine, which may be connected or disconnected at will to shaft W. The lever J operates a shaft R in Fig. 3404, which connects (by a rod corresponding to rod K in Fig. 3403) to each link motion; hence all the links reverse together, and the ratio expansion of one cylinder to the other cannot be varied, or in other words, the point of cut off will be alike for each cylinder, let the link motion be shifted to whatever position it may.

The beam S, Fig. 3403, for working the air, circulating and feed pumps, is driven from the cross head of the intermediate cylinder.

The boilers are of the Scotch pattern that is usually employed for high pressures, as 160 or more lbs. per square inch, and have Fox corrugated furnaces and stay tubes.

Each cylinder requires a starting valve (which is sometimes called an auxiliary valve or a bye pass valve), which is used to warm the cylinder before starting the engine, and also (when there is no vacuum in the condenser) to admit high pressure steam when the high pressure piston is on the dead centre, in which case, there being no vacuum and no admission of steam to the low pressure cylinder, the engine would not have sufficient power to start.

In some cases the high pressure cylinder has no starting valve, the reversing gear being used to admit steam to one end or the other of the high pressure piston, and the starting valve being used to admit enough live steam to the low pressure cylinder to compensate for the absence of the vacuum.

When the vacuum in the low pressure cylinder is maintained while the engine is standing still, its starting valve obviously need not be used, except for warming purposes, before starting the engine; as soon, however, as the engine has started, the starting valve must be closed.

Each cylinder is provided with a relief valve, both at the top and at the bottom, to relieve the cylinder from a heavy charge of water, such as may occur if the boiler primes heavily.

Each cylinder is also provided with drain cocks, to permit of the escape of the ordinary water of condensation in the cylinders when the engine is started, and also for use if the boiler primes.

The low pressure relief valve also prevents the accumulation of too great a pressure in the low pressure cylinder, which, from its large diameter, is not strong enough to withstand high pressure.

The oiling apparatus for the cylinders is arranged as follows:

In some cases pumps, and in others automatic or self-feeding devices are used. Oil is fed to the steam pipe of the high pressure cylinder, and this lubricates both the valves and the cylinders, but in many cases it is also fed to the steam chest, so as to afford more perfect lubrication to the valve.

For the low pressure cylinder the oil is fed into the receiver, and usually at a point near the slide valves.

Large marine cylinders are usually constructed with a separate lining, which may be replaced when worn or otherwise required.

A surface condenser consists of a cast iron shell or chamber forming the back of the engine frame. At each end of this chamber is a short partition, so that the condenser is divided lengthways into what may be called three compartments, of which the middle one is the longest and contains a number of thin brass tubes about 5/8 or 3/4 inch in diameter, the ends of these tubes being held in the plates or tube sheets forming the partitions. The object of providing tubes of small diameter is to obtain a large area of cooling surface.

The exhaust steam from the engine generally passes into the shell or body of the condenser, filling the middle partition and surrounding the tubes.

The condensing or circulating water passes through the tubes, and by keeping them cool condenses the steam and forms a vacuum or partial vacuum in the condenser, which, having open communication with the low pressure cylinder, therefore gives a corresponding degree of vacuum on the exhaust side of the low pressure piston.

In some designs, however, the steam passes through the tubes and the circulating water fills the middle compartment of the condenser. As, however, there is no pressure to counterbalance the weight of the water, it is preferable to have the water inside the tubes, so that they are subjected to a bursting pressure, in which case they may, for a given strength, be made thinner, because the strength of the tube to resist bursting is greater than its strength to resist collapsing, hence the circulating water usually passes through the tubes. The chamber at the ends of the condenser permits the water to distribute through all the tubes.

In some cases the chamber at one end is divided horizontally into two compartments, so that the water is compelled to pass through one half and return through the other half of the tubes.

The water of condensation falls to the bottom of the condenser, from which it is removed by the air pump, which delivers it to the hot well.

The hot well is situated on the side of, and extends above, the pump, whose upper end it covers, thus water sealing the top of the air pump and preventing air from passing into it through a leaky valve or bucket.

The top of the hot well is provided with a _vapor pipe_, which permits the air and gases to pass overboard. This pipe emerges through the side of the ship above the water line, and as there is no valve between the hot well and the sea, no pressure can possibly accumulate in the hot well.

The boiler feed is taken from the hot well either by the feed pump or by injectors, as the case may be.

In case the boiler feed should stop working, however, the hot well is provided with a pipe of large diameter, and called the overboard discharge pipe, so that the water of condensation may not accumulate a pressure in the hot well if the boiler feed ceases.

This overboard discharge pipe is provided with a weighted valve (placed at the side of the ship), which is constructed after the manner of a safety valve, relieving the hot well of pressure if the water accumulates, and preventing the sea water from entering the hot well.

To prevent loss of fresh water, the exhaust steam from the various engines and pumps (if any) about the ship passes to the condenser and is pumped into the hot well.

In some cases, however, a separate and independent condenser is used for the smaller engines about the ship.

An independent condenser is one whose air pump and circulating pump are not worked from the main engine, and can therefore be operated when the main engine is standing still.

If the main condenser is independent, it may be started so as to form a vacuum before the main engine is started, and thus obviate the use of the starting valve on the low pressure cylinder except to warm the cylinder before starting.

Feed water for the boilers when the engine is standing is obtained by a pipe from the bottom of the condenser, so that the water of condensation of steam blown through the engine cylinders, and from the exhausts from the smaller engines about the ship, may be pumped or forced direct from the bottom of the condenser to the boiler.

This feed from the bottom of the condenser is necessary when the air pump is not working, and the water of condensation is not pumped into the hot well.

If the water thus obtained is not enough to keep the boilers supplied, an auxiliary or salt water feed admits extra water from the circulating water to the inside of the condenser to supply the deficiency.

This secondary suction pipe is provided with a valve because it must be shut off before the engine is started.

All the drain pipes from the cylinder pass into the condenser so as to save the fresh water.

The air pump is usually worked by a beam, receiving motion from the cross head of the low pressure cylinder.

The circulating pump is usually worked by the same beam as the air pump, or receives its motion from some other part of the main engine. In some cases, however, an independent circulating pump is employed.

It receives its water from a pipe leading to the sea, which is provided with an injection cock or Kingston valve, placed close to the side of the ship and well below the sea level. This valve is used to shut off the circulating water and prevent its flooding the ship in case of accident to the condenser or circulating pump.

The circulating water, after passing through the condenser, discharges overboard through the circulator discharge pipe.

This pipe is also provided with a valve placed close to the ship's side, at or above the water level, so that the opening at the ship's side may be closed, and sea water prevented from entering the ship in case of breakage to the condenser, etc.

To enable a surface condenser to be used as a jet condenser in case of accident to the circulating pump, a pipe leads from the injection cock of the circulating supply pump into the bottom of the exhaust pipe or column, where it enters the condenser.

This pipe is supplied with a spray or rose nozzle, which divides up the injection water and causes it to condense the steam as it enters the condenser.

An additional pipe is sometimes added to the suction side of the circulating pump, for use in pumping out the bilge by means of the circulating pump in case of emergency, and also for pumping out ballast tanks when the vessel is provided with such tanks.

An air valve is sometimes fitted to a reciprocating double acting circulating pump. It admits air to the water during the up stroke of the pump, and closes on the down stroke. The air thus admitted acts as a cushion to soften the shock of the water.

A snifting (or snifter valve, as it is sometimes called) is a valve fitted to the condenser and that opens upwards to permit of the discharge of the air and gases before the engine is started. It also serves to prevent any water from leaky condenser tubes from filling the condenser and flooding the engine cylinders. It is so loaded with dead weight that it opens automatically when the water in the condenser has reached a certain height and must be placed as low down on the condenser as possible, so as to receive the weight of the full height of the water in the condenser.

Condenser tubes are made water tight in the tube plates of the condenser by wooden or sometimes paper ferrules, which fit the tube and drive into the tube plate. In other cases, however, the tube ends project through the plates, and a rubber washer is placed on the end of each tube. A covering plate is then bolted over the whole of the tube ends, the holes in the covering plate being parallel for a short distance, and then reduced in diameter so as to form a shoulder. The rubber rings compress and make a joint, and the shoulders prevent the condenser tubes from working out endways from expansion and contraction. The tubes are usually about 3/64 inch thick.

Fig. 3404.]

A blow through valve is a valve attached to the casing or steam chest, and connecting by a pipe to condenser to blow out the air and gases that may have collected there when the engine is standing still, and that also connects to the exhaust port of the high pressure cylinder, so as to supply live steam to the low pressure cylinder in case the high pressure cylinder should get disabled.

A bucket air pump is one in which there is a valve or valves in the pump piston, hence the pump is single acting, drawing on the lower side of the piston and delivering on the upper, hence the capacity of the pump per engine revolution is equal to the diameter of the bucket multiplied by the length of its stroke. The suction or foot valve is at the foot of the pump, and the delivery valve at the head.

A piston air pump is double acting, since it draws on each side alternately of the piston, one side delivering while the other is drawing, hence two suction and two delivery valves are required.

A plunger air pump is one in which a plunger is used in place of a piston, the delivery being due to the displacement of the plunger.

An air pump trunk is a hollow brass cylinder attached to or in one piece with the piston or bucket of the air pump. The rod which drives the piston passes through the trunk, and connects to a single eye at the bottom of the trunk.

A trunk air pump is necessary when the pump rod is driven direct from the crank shaft, and therefore has sufficient lateral motion to push the pump piston sideways, which would cause friction and excessive wear to the gland that keeps the trunk tight. The delivery capacity of the pump is obviously diminished to an amount equal to the displacement of that part of the plunger that passes through the gland and within the pump bore, whereas in a piston pump the delivery capacity is only diminished to an amount corresponding to the displacement of the pump piston rod.

A bucket pump may in some cases be worked without either a foot or a head valve, since the bucket valve will answer for both in cases when the delivery water cannot pass back into the pump on the down stroke of the bucket.

It will, however, be more efficient with the addition of either of them, and most efficient with both.

A bucket pump with a foot valve and no discharge valve would, however, suffer more from a leaky gland than if it had a discharge valve and no foot valve, because the air would, on the ascent of the bucket and the closing of the bucket valve, pass to the suction side of the bucket and impair the vacuum.

Let the delivery valves be where they may, the foot valve will always have some water above it, and the pump bucket will dip into this water, and on lifting produce a vacuum that will cause the pump to fill with water. Notwithstanding that the gland may leak air on the other side of the bucket, this air will in a single acting pump be expelled with the water, but in a double acting pump it will impair the vacuum, and therefore the suction, on the gland side of the piston.

Bucket air pumps are provided with a valve or pet cock on the top or delivery side of the bucket and above the bucket, when the latter is at the highest point of its stroke. This valve opens on the descent of the bucket, admitting air to act as a cushion between the surface of the water and the delivery valve, when the water is about to meet the latter. It obviously reduces the effectiveness of the pump, and in a double acting pump is inadmissible, because of its impairing the vacuum and the suction.

This valve also enables the engineer to know whether the air pump is working properly.

A pet cock is also supplied to the feed pumps for this same purpose.

A bilge injection is one in which the injection water is taken from the bilge, which may be done when the ship makes more water than the bilge pumps can get rid of.

The fittings necessary for a bilge injection are a cock or globe valve placed on the side of the condenser, and at or near the foot of the exhaust pipe, with a spray or rose inside that pipe. From the cock a pipe, usually lead, leads to the bilge, having at its end a strainer or strum, and care must be taken that this strum does not get choked and let the condenser get hot from the exhaust steam not being condensed.

The water in the hot well of a surface condenser is usually kept at a temperature of about 100° Fahrenheit. A higher temperature than 100° Fahrenheit injures the rubber valves of the air pump, while lower temperatures cool the engine cylinders too much and cause waste from cylinder condensation. Moreover, it is obvious that, since the boiler feed is taken from the hot well, it is desirable to keep it as hot as the valves and as the desired degree of vacuum will permit.

An air vessel or air chamber is a vessel fitted to the delivery and sometimes also to the suction side of a pump. Its office is to maintain a steady flow of water through the pipes.

Thus, in the case of the delivery air chamber, when the pump piston is travelling at a speed above its average for the stroke, the water accumulates in the air chamber, and the air is more compressed, while, when the pump is on the dead centre, or at the end of its stroke and the delivery valve closes, the air compressed in the air chamber continues the delivery or discharge, thus maintaining a more uniform flow.

Pumps sometimes have an air or vacuum chamber on the suction side, from which the air is exhausted when the pump starts, leaving a vacuum which causes a steady flow of water up the suction pipe.

Both these chambers are more effective as the speed of the pump increases. The chamber on the delivery side is apt to lose its air, which is gradually absorbed by the water, which should be let out when the pump is standing still.

Feed escape valves or feed relief valves are fitted to the feed pumps, so that in case all the feed water cannot pass into the boiler it may pass back to the hot well.

The construction of a feed escape valve is as follows:

It is an ordinary mitre valve held to its seat by the compression of a spiral spring, whose pressure upon the valve may be regulated by an adjusting screw, whose end abuts upon a stem provided for the purpose.

In proportion as the valve is relieved of the pressure of this spring, a greater proportion of the water delivered by the feed pump will pass back into the hot well, hence the amount of boiler feed may be regulated by the feed escape valve, which also acts as a safety valve, preventing undue pressure in the feed pipe.

When no feed escape valve is employed, the delivery water from the feed pump must pass unobstructed to the boiler, or the feed pipes may burst from over pressure, and it follows that the feed check valve on the side of the boiler must not be restrained in its amount of lift, hence it must not have a lift adjusting screw.

The amount of the boiler feed must, in this case, be regulated from the suction side of the pump, the suction pipe being fitted with a cock or valve whose amount of opening may be adjusted so as to regulate the amount of water drawn per pump stroke from the hot well.

If the feed valve on the suction side, or the escape valve on the delivery side of the pump, as the case may be, is adjusted to permit of a proper amount of boiler feed, and yet the feed is insufficient or ceases altogether, it may occur from the following causes:

1st. From the suction valve sticking or being choked, or from the delivery valve being choked and not seating itself, thus either letting the suction water pass back into the hot well, or the delivery water pass back into the pump.

2d. Through leaks in the joints of the pump or of the suction pipe.

3d. From the water in the hot well being too hot.

4th. Through the spring of the escape valve having become disarranged.

5th. If two or more boilers are connected, and one has less pressure in it than the other, it may take most of the feed water, or the water of the other may empty itself into it.

Bilge Injection. The injection water for a common or jet condenser may be obtained in one of two ways: first, direct from the sea, which is that for ordinary use; and secondly from the bilge, which is resorted to to assist the bilge pump in cases of emergency.

The necessary fittings for a bilge injection are, a pipe leading from the condenser to the bilge, with a cock at the condenser end and a strainer at the bilge end.

This pipe should be fitted with a check valve, which opens by lifting upwards so that no water can pass down it into the bilge, or otherwise, if the main and bilge injections should happen to be left open together, the water from the main injection might pass down into the bilge. This check valve should be so constructed that its amount of lift can be regulated and as much of the bilge water used for injection as the circumstances may require.

In the case of surface condensers, the bilge water is drawn off by the circulating pump and used to supplement the main circulating water. The pipe from the bilge in this case leads to the suction side of the circulating pump, and requires a strainer at the bilge end, a cock at the circulating pump, and a check valve.

A ship's side air pump discharge valve is an ordinary dead weight mitre valve that opens to let the water pass out into the sea, but seats itself and closes if the water attempts to pass inwards. It differs from a common stop valve in being weighted, and therefore self-acting. It requires to be lifted before starting the engine, as such valves are liable to stick in their seats.

The course of the main injection water of a jet condenser is as follows: From the rose plate or strainer, through the injection valve and pipe to the condenser, where it mingles with the exhaust steam and from which it is pumped with the products of condensation into the hot well. From the hot well it passes mainly overboard through the Kingston valve, but that part of it used for the boiler feed passes through the suction pipe and valve into the pump, and thence through the delivery valve, pipe and check valve into the boiler.

The course of the main circulating water of a surface condenser is through the Kingston valve (on the ship's side or bottom), and the circulator inlet pipe, either direct to the condenser, from which it is _drawn_ by the circulating pump, or else it passes through it, and is _forced_ through the condenser. It circulates through the condenser twice or thrice according to the construction, and is forced overboard by the action of the circulating pump, passing through a valve on the ship's side or bottom.

The advantages of surface condensation are, first, that the feed water is obtained at a higher temperature than if injection water was fed to the boiler. Second, the feed water is purer, and therefore less water requires to be blown out of the boiler in order to keep it clean. Third, the boiler does not scale so much, hence its heating surface is maintained more efficient; and fourth, the boiler suffers less from expansion and expansion strains when hot feed water is used.

Surface condensers foul from the grease with which the cylinders are lubricated and from the salt in the injection water. The condenser is cleaned by the admission of soda with the exhaust steam and by washing out.

A condensing engine has the following cocks and valves on the skin of the ship in the engine room: The main Kingston valve for the injection, or circulating water, the main delivery valve from the condenser, the bilge delivery valves, and the water service cocks for keeping the main bearings of the engine cool with streams of cold water.

A donkey engine is a small engine used to feed the boiler, and has the following connections: A steam pipe from the boiler to drive the donkey engine; and exhaust pipe into the condenser; a suction pipe from the hot well or from the sea, as the case may be; and a delivery pipe to the boiler; a suction pipe from the bilge, so that the donkey pump can assist in pumping the bilge out; a suction pipe to the condenser, to circulate the water when the main engines are stopped, and thus maintain the vacuum; and a suction pipe from the water ballast tanks, to pump them out when necessary.

The pipes that lead from, or go to, the sea are: Boiler blow off pipe, sea injection or circulator pipe, condenser discharge pipe, and, in some cases, donkey feed suction pipe.

The parts of an engine that are generally made of wrought iron are those in which strength with a minimum of weight and size is desired; for example, the piston rod, cross head, connecting rod, crank shaft, crank, eccentric rods, link motion, valve spindle pump rods, and all studs, bolts, and nuts.

The parts generally made of cast iron are those where strength and rigidity are required, and which are difficult to forge, while weight or size is of lesser importance, such as the bed plate, cylinders, pistons, condensers, and pumps.

The parts sometimes made of steel are those subject to great wear, and for which strength with a minimum of size is necessary, as piston springs, piston rods, connecting rods, cranks, crank pins, and valve rods.

The parts generally made of brass are those subject to abrasion or corrosion, as the connecting rod brasses, the bearings for the crank shaft, the pump plungers or pistons, and their rods, linings for the pump barrels or bores, the bores of the glands, the condenser tubes, and all cocks and valves.

White metal or babbitt metal is sometimes used in place of, or in connection with, brasses, serving as an anti-abrasion surface. It is easily renewed, as it is cast into its place, but will melt and run out at a temperature of about 600° Fahrenheit.

Muntz metal is used where iron or steel would suffer greatly from corrosion when in contact with salt water. It can be forged.

The difference in the composition of cast iron and steel has never been determined; the difference lies in the percentage of carbon they contain and the structure of the metal. Cast iron will not weld.

Cast iron is brittle, of granular structure, and always breaks short, having a very low elastic limit.

Wrought iron is tough and fibrous, will weld but will not harden, and is stronger than cast iron.

Steel is stronger than wrought iron, and will weld and harden and temper. The breaking strain of wrought iron varies from about 42,000 to 60,000 lbs. per square inch of section.

Steel is tempered by first being heated red hot and suddenly cooled (usually by plunging it into cold water), which hardens it. The surface is then brightened, and on being reheated the tempering colors appear, beginning at a pale yellow, and deepening into red, brown, purple, and blue, the latter gradually fading away as the metal is re-heated to a red heat. The higher the temperature to which the hardened steel is reheated the softer or lower it is tempered.

These colors merely indicate the temperature to which the piece is reheated, since they will appear on steel not hardened and upon iron.

Case hardening is a process that converts the surface of wrought iron into steel, which is accomplished by placing them in a box filled with bone dust, animal charcoal, or leather hoofs, etc. The box is sealed with clay, heated red hot for about 12 hours, and the pieces are quenched in water.

The parts usually case hardened are the link motion, and other light working parts that are of wrought iron.

The forgeable metals used in engine work are wrought iron, steel, copper, and Muntz metal. The brittle or short metals are cast iron and brass.

Welding is the joining of two pieces solidly together. Wrought iron, steel, and Muntz metal can be welded.

All the metals used in the construction of marine engines expand by heat, and this is allowed for in adjusting the lengths of the eccentric rods, or of the valve spindles when setting the valve lead. In the case of two marine boilers being connected together, the steam pipe is fitted with an expansion joint, one pipe end having an enlarged bore to receive the other. The joint is made by packing, which is squeezed up by a gland, whose bore fits on the outside of the pipe which moves through the gland bore, from the expansion and contraction.

The piston of a marine engine steam cylinder is a disc of cast iron, into which the piston rod is secured. Its body is cored out to lighten it. Around its circumference is a recess to receive the packing ring or rings, each of which is split across so that it may be expanded (to fit the bore of the cylinder) by means of the packing or of the springs. The split is closed in the centre by a tongue piece let into the ring, and fastened to one end of the ring.

To hold the piston rings or ring in place, a junk ring is employed, being an annular ring bolted to the piston. The piston rings are set out to fit the cylinder bore by suitable springs. The round plugs seen on the piston face merely fill the holes used to support the core in the mould and to extract it from the finished casting.

Cylinder drain cocks sometimes have a check valve upon them, so that while the water may pass out of the cylinder the air cannot pass in and destroy or impair the vacuum.

Cylinder escape or relief valves are provided at the top and at the bottom of the cylinders, and consist of a spring loaded valve with an adjusting screw to regulate the pressure at which they shall act. They are most needed when the boiler primes heavily, and the water might knock out the cylinder heads or covers. They should be enclosed in a case with a pipe to lead the water away, thus preventing it from flying out and scalding the engineer.

A link motion is a valve gear by which the engine may be reversed (caused to run in either direction), or which may be used to vary the point of cut off. The advantage of the link motion is its simplicity and durability.

A link motion for a marine engine is usually of the Stephenson type, and consists of two eccentrics or eccentric sheaves fixed upon the crank shaft, and so set as to give more lead at the bottom than the top ports, because the wear of the journals, brasses, and pins gradually increases the lead at the upper, and correspondingly diminishes that at the lower port. In addition to this, however, more lead is required at the bottom port, to counterbalance the weight of the piston at the end of its descending stroke. The eccentric hoops or straps drive the rods which connect to the ends of the link.

The link may be a curved, solid, or a slotted bar, and in either case has fitted to it a block or die which connects to the valve spindle.

The link is pivoted at its centre to a swinging arm or suspension link,[58] and by this arm may be moved endways to bring the required end of the link beneath the valve rod or spindle. From the positions in which the eccentrics are set, one end of the link operates the valve to go ahead, while the other end operates it to go astern; hence all that is necessary (so far as the link motion is concerned) to reverse the engine is to move the link endwise to the requisite amount, which, for full gear, is so that the block is at or near the end of the link.

[58] See page 383 for the construction of a link motion.

In proportion as the link block is (by moving the link endways) brought nearer to the middle of the link, the valve travel is reduced and the point of cut off is hastened, thus increasing the expansion.

When the link block is in the middle of the link, the latter is in mid gear, and the valve only opens the ports to the amount of the lead, and the link action is the same, whether the engine moves backwards or forwards.

The motion of the link is as follows:

The two ends are vibrated by the eccentrics from the central pin of the link hanger (or suspension link) as a centre of motion, while at the same time this end of the link hanger swings in an arc of which its other end is the centre of motion.

In small engines the link is sometimes used for varying the expansion as well as for reversing the direction of engine revolution.

In large engines it is used for reversing only, a separate expansion valve being used for varying the point of cut off.

In small engines the link is moved endwise for forward or backward gear by a simple arrangement of hand levers. In large engines these levers are supplemented by a worm and worm gear, and in still larger engines a steam reversing gear is used for shifting the links from forward to backward gear, or vice versa.

When there is no link motion, a Joy valve gear, a Marshall valve gear, or a loose eccentric may be used. A loose eccentric is one that can be moved around the shaft to reverse the engine. It may be moved around the shaft by mechanical means, or the eccentric rods may be disconnected, and the valve worked by hand, to cause the engine to run in the required direction, until a pin fast in the shaft meets a lug on the eccentric and drives it, there being two such lugs or shoulders spaced the requisite distance apart on the eccentric. This plan is obviously only suitable for small engines.

A separate expansion valve is a valve employed to effect the cut off and vary the expansion. It does not affect either the admission or exhaust of the steam to the cylinder.

It is used because by its means an early point of cut off and high rate of expansion may be obtained with a fixed point of exhaust, a fixed amount of compression, and a fixed amount of lead, whereas with the link motion alone the exhaust occurs earlier in the stroke, and the compression and the lead increase as the link is moved from full gear towards mid gear. The expansion valve should, when the engine is to be started, be set for the latest point of cut off. The eccentric for the expansion valve is set opposite to the crank, in order that its action may be the same, whether the engine runs backward or forward.

The small cylinders on top of the steam chests are for the purpose of guiding the upper ends of the valve spindles, and are fitted with pistons having steam beneath, the space above being in communication with the condenser. The steam pressure on the piston supports the weight of the valves and valve gear.

The friction of a slide valve may be relieved or reduced by excluding the steam from its back, which is done by various means, such as by a ring cast on its back and working steam tight against a plate held independently of the valve. The interior of the ring should be open to the exhaust.

The friction of a slide valve is caused by the steam pressing it to its seat, the amount of this pressure varying with the fit of the valve to its seat, and its position over the ports, or, in other words, upon how much of the valve area has steam pressing on one side only.

The travel of the eccentric rod is the distance it moves measured on a straight line. It is equal to twice the throw of the eccentric.

The throw of an eccentric is the distance between the axis of its bore and the centre or axis from which its circumference was turned in the lathe.

Double beat valves are composed of two discs or mitre valves, one above the other on the same stem, so that as the steam presses on the opposite faces of the two discs the valve is balanced. The objection to their use as safety valves is, that they are balanced and would not lift unless the area of the upper disc was made larger than that of the lower one, in which the objection would remain that the two discs do not expand equally, hence they are apt to leak. They are sometimes used instead of slide valves, but are objectionable because a separate admission and exhaust valve is required at each end of the cylinder, and because at quick speeds of revolution they fall to their seats with a shock or blow which wears out both the valve and the seat. When a high piston speed is obtained by great length of piston stroke, and not by high rotative speed, their use is less objectionable.

Expansion joints are joints which permit the parts they connect to expand and contract without straining them. They are necessary on the steam pipe connecting one boiler to another, and on the main steam pipe from the boilers to the engine. The working surfaces require to be of brass, so that they will not corrode.

They require the collar on the internal pipe of the joint (on which the gland fits) to be permanently fixed by soldering or brazing, and check nuts on the studs, so that the internal pipe shall not be blown out from the steam pressure.

This pipe is also sometimes fitted with chains or stops, in case the studs should break, or the nuts or collar strip.

An oil cup is either a cavity cast in the piece or a cup shaped vessel or hollow cylinder screwed in. It contains a pipe extending up about three-fourths of its height, and through this pipe the oil is fed to the surface required to be lubricated. A hinged lid or, in some cases, a screwed cap covers the oil cup to exclude dust, etc.

The syphon or worsted consists of a number of threads of worsted or lamp wick of equal lengths; a piece of lead or copper wire is laid across the middle of the worsted, the copper wire is doubled and twisted and is then pushed down the tube, carrying the doubled end of the worsted with it. The upper ends of the wire are bent over the end of the tube so as to hold the worsted, whose lower end should pass down below the level of the bottom of the oil cup. The oil feeds (on the syphon principle) through the medium of the wick or worsted, which should not fit the tube tight but quite easily, its upper ends hanging over the top of the tube to the bottom of the cup.

The worsted may be cleaned with scalding water, or by water thrown upon it from the boiler.

Tallow cups for high pressure cylinders must have two cocks, so that after the cup is filled the top cock may be closed and the bottom one then opened. The top cock prevents the tallow or oil from being blown out of the cock by the steam. For the low pressure cylinder a cup with a single cock will answer, as the cock may be opened when the vacuum is at that end of the cylinder, and the air will force the oil or tallow in.

A steam lubricator or impermeator is an automatic oil feeding device placed on the steam pipe of the high pressure cylinder. Steam lubricators are made in various forms, some having a positive feed by a pumping arrangement, while in others the oil floats upon water in the body of the lubricator to which steam is admitted; the condensation of the steam increases the quantity of water and causes the floating oil to overflow and feed through a pipe leading into the steam pipe or steam chest, as the case may be. Cooling the impermeator causes more rapid condensation, and increases the amount of oil fed to the steam.

Cylinder escape or relief valves do not let all the water out of the cylinder because of the clearance,[59] hence the amount of water left in will equal the amount of clearance.

[59] See page 372, on clearance.

The small cylinders on top of the steam chest are for the purpose of guiding the upper ends of the valve spindles, and are fitted with pistons having steam beneath, the upper end being in communication with the condenser.

The effort of the piston to rise supports the weight of the valves and valve gear.

The valves of a marine engine that are worked by hand are, the stop valves for letting on steam from the boiler, the safety valve, which is lifted to see that it is in proper working order, the Kingston valve for letting in the circulating water, the blow through or starting valve for warming the cylinders and starting the engines. The valve for adjusting the rate of boiler feed has its lift adjusting screw operated by hand. The slide valve may also be operated by hand before the engine is started, or it may be operated by a steam reversing gear. The expansion valves are also set by hand to regulate the point of cut off or amount of expansion. The valves that are operated automatically, or from the motion of the parts, are the slide and expansion valves, the suction and delivery and check valves of all pumps, the air pump bucket valves, the snifting valves, and the ship's side overboard discharge valves. When the engine is stopped and the steam shut off, close the dampers to check the draught and open the drain cocks on the high pressure cylinders.

If the engine is soon to start and the pressure in the boiler is at the blowing off point, start the boiler feed, if the height of the water in the boiler will permit it, and this is a good time to clean the fires. If the engine is to stop for any length of time, shut off the impermeator and the injection supply.

A vacuum gauge is an instrument for measuring the total or absolute pressure, or pressure above a perfect vacuum, and it is used to indicate the degree of vacuum that exists in the condenser, which, when the various joints about the cylinder and condenser are tight, averages about 27 inches of mercury when the temperature in the hot well is about 100° Fahrenheit.

In round numbers a column of mercury 32 inches high equals the weight of the atmosphere,[60] hence taking the weight of the atmosphere at sea level to be 15 lbs. per square inch, then each two inches of mercury represents an atmospheric pressure of 2 lbs. Suppose then that a bent U shaped tube, each leg of which is 30 inches high, is half filled with mercury, and that one end is in communication with the condenser, and the other end is open to the atmosphere, and if there was a perfect vacuum in the condenser, the pressure of the atmosphere in the open leg would force all the mercury into the leg that communicated with the condenser, hence there would be a column of 30 inches of mercury in one leg, and air in the other.

[60] See "Barometer," Chapter XL.

If there was in the condenser a pressure of 1-1/2 pounds per square inch above a perfect vacuum, the mercury would stand 27 inches high in one leg, and 3 inches in the other, and so on, hence from the height of the column of mercury above its natural level the degree of vacuum in the condenser may be known. But the pressure of the atmosphere varies with its temperature, and the weight of mercury also varies with its temperature.

To find the total pressure in the condenser, therefore, we subtract height of the column of mercury given by the condenser from the height of the column in the barometer, and divide the remainder by 2.

_Examples._--The barometer stands at 29.5 and the vacuum gauge at 26, what is the absolute pressure in the condenser?

Here,

29.5 - 26 = 3.5 ÷ 2 = 1.75
Answer, 1-75/100 lbs. per square inch.

A dial vacuum gauge of the Bourdon construction is similar to the Bourdon steam gauge, that is used upon the boiler, except that the inside of the elliptical tube is in communication with the condenser and the atmospheric pressure bends the tube into a curve of smaller radius (instead of to a larger one, as in the case of the steam gauge).

Obviously, therefore, the zero of the dial vacuum gauge is atmospheric pressure.

Suppose the dial vacuum gauge shows 10 lbs., the steam gauge 120 lbs., and the barometer 15 lbs., and we may find the total pressure or pressure above vacuum of the steam in the boiler is as follows:

One-half Pressure by steam gauge = 60 lbs.
A perfect vacuum = 15 lbs.
--
Total pressure supposing condenser had a perfect vacuum = 75 lbs.

To make the correction necessary because there is not a perfect vacuum in the condenser, we then proceed as follows:

Barometer 30 inches of mercury = 15 lbs. per sq. in.
Dial vacuum gauge = 10 " " " "
--
Actual pressure in condenser = 5 " " " "

Then

Total pressure supposing condenser had a perfect vacuum = 75
Actual pressure in condenser = 5
--
Actual pressure of the steam = 70

Racing means a sudden acceleration of the engine speed, and occurs when the propeller is not fully immersed in the sea, as by reason of the pitching of the ship. Racing augments the strain on the working gear of the pumps, and is likely to lead to accident. It is obviated by the use of a governor or by partly shutting off the steam by hand.

A marine governor is a device for controlling the engine speed, by reducing the supply of steam to the engine cylinder whenever the engine begins to race. The governor is driven by band or rope on the crank shaft. Governors are made in various forms; thus, in one the shaft has a fly wheel and a friction clutch, one half of which is fast on the governor shaft, while between it and the other is a spiral spring which connects the two halves. If the speed accelerates, the sliding half of the clutch is moved along the governor shaft, and by means of links it closes the throttle valve of the main steam pipe, thus wire drawing the steam, reducing its pressure and thereby controlling the engine speed.